In the early days of the US Postal Service’s national airmail service, pilots had to navigate across the USA by sight alone – a task that bad weather could make extremely difficult. And so a network of towers was built, each bearing a gas-powered light for night-time visibility, and each with a large arrow-shaped foundation designed to assist daytime navigation. Almost all of the ~1,500 towers were dismantled long ago, but a number of the concrete arrows exist to this day, such as in the front yard of this farmhouse in Minnesota. The development of the beacon system…